Up to 100 Indonesian men, believed to have been hired by a palm oil firm, took a team of official environmental investigators hostage on Friday and threatened to burn them alive, Indonesia’s environment ministry has said.

The government team of seven were documenting illegal forest fires, which are often set ablaze deliberately by agriculture firms to clear land for replanting during the dry season.

The ministry of environment and forestry said its team was held overnight until Saturday morning when negotiations with police and local officials led to their release. During their detention, the investigators were told they would be killed and dumped in a river, the ministry added.

It said there were “strong indications” that their captors were mobilised by Andika Permata Sawit Lestari (APSL), a palm oil firm operating in Riau province, along the Strait of Malacca.

“With this incident, the investigation of [APSL] will become our top priority,” the environment minister, Siti Nurbaya, said in a statement.

“The environment ministry will investigate this and take strict action in accordance with the law,” she added.

The Guardian was not immediately able to contact the company. But the Jakarta Post reported that APSL denied defying the ministry, blaming the incident on local farmers who were offended by the investigators. “We are mentioned every time something happens on the local farmers’ lands. We don’t facilitate the local farmers. We are not in the position to defy the ministry,” a spokesperson said.

“The local farmers had spoken to the media. It was not a hostage situation. It was their own spontaneity because they felt they were not respected.”

Every year, fires destroy wildlife and the acrid smoke from illegal burns across Indonesia fills the sky, causing deadly respiratory issues across the region.

Home to the world’s third-largest area of tropical forests, Indonesia is also the world’s fifth-largest emitter of the greenhouse gases, largely due to deforestation and burning.

Some of the fires are seasonal with trees burning naturally towards of end the dry season. But most of the smoke is caused by illegal slash and burn practices, where land is set on fire as a cheap way to clear it for farming. In particular, peat soil is burned to make the ground ready for palms.

The country has vowed to crack down on the fires and regularly engages in diplomatic spats with neighbouring Malaysia and Singapore, where the smoke causes schools in both countries closed due to the low air quality.

The issue is so severe that south-east Asian countries set up an intergovernmental agreement. Since its inception in 2002, the haze has continued annually.

President Joko Widodo in April said he would impose a moratorium on expanding palm oil plantations, which produce oil for products such a soap and chocolate, part of the country’s efforts to reduce the impact on the environment.

Under Indonesian law, management of companies allowing illegal fires can face up to 10 years in jail. But the country’s huge size and the local clout of powerful agribusiness firms make it difficult to police companies engaged in slash and burn techniques.

Korean palm oil firm accused of illegal forest burning in Indonesia

Read more

Before they were let go, the team held on Friday agreed with their captors to erase photos on their camera, the ministry statement said. The team had found that more than 2,000 hectares (4,942 acres) of forest had been burned by workers of the company, it added.

The statement said that the investigators were still able to use drone footage, which it said showed thousands of hectares of forest had been burned illegally.

“As far as the eye can see, an area that was once peatland has been converted into oil palm plantation,” Nurbaya said.